When drilling through subterranean formations in the exploration for oil and gas, it is common practice to drill larger diameter holes at the surface, and successively smaller diameter holes as the well is drilled deeper, cementing tubular casings in place at various depths along the well bore. It is often desirable, however, to drill a hole larger than the inside diameter of the last casing that was set, at some known depth below the surface. Since conventional drill bits large enough to generate the desired well bore diameter will not fit inside the casing that has already been set, special tools are used to drill a well bore larger in diameter than the inside diameter of the casing. One such tool used for this purpose is a bi-center bit.
A bi-center bit is an undersized drill bit with a large eccentric cutting structure located off-center above a smaller pilot drill bit that is centered axially with the drill collars. The bi-center bit is sized so that while being run into the hole, the pilot bit is pushed to one side to allow the tool to pass through the inside of the casing. Once at the bottom of the hole, though, the pilot bit then acts as a centered pivot point for the eccentric cutting structure above, which generates a hole larger in diameter than the inside diameter of the casing through which it passed.
Despite their widespread use, many bi-center bits suffer from one or more limitations. One such limitation is the inability of many bi-center bits to drill out cement or casing shoes. This is due to the fact that when the bit is inside a casing, the pilot section of the bit tends to rotate around the center of the drill string, causing the gauge cutters to engage the casing. This damages both the cutters and the casing. Additionally, since the center of the pilot bit is aligned with the drill string, the bit also tends to rotate off-center when inside the casing. This can cause damage to the cutters on the leading face of the bi-center drill bit. The extent of this damage may be further increased when a directional drilling bottom hole assembly is attached to the drill string just above the bit.
Another limitation of many bi-center bits is that cutters placed in the center of the bit may rotate backward (i.e., opposite their cutting faces) when the bit is inside a casing. This backward rotation prevents efficient cutting action, and when the cutters contact the casing, may result in damage to the cutters.